Myocardial injury after ischemia and subsequent reperfusion is mediated by oxygen-derived free radicals species such as the superoxide anion radical, hydrogen peroxide, and the hydroxyl radical. Early reperfusion of ischemic myocardium in an evolving infarct can improve cardiac function and overall survival. However, evidence also exists showing that reperfusion leads to a burst of oxygen radical formation which further injures the tissue, and is supported by the observation that significant quantities of these radicals are generated during the post-ischemic period. Administration of antioxidant enzymes such as superoxide dismutase (SOD), can limit reperfusion injury, but some controversy remains as to the extent to which antioxidants protect the heart. Three highly compartmentalized forms of SOD exist. Cytosolic (Copper-Zinc SOD), mitochondrial (Manganese-SOD), and an extracellular or secreted (EC-SOD) SOD are all distinct chromosomal gene products. In this study, we will employ an isolated working murine heart preparation to first characterize a time course dependent ischemia reperfusion injury in murine hearts. We will subsequently use this model to carry out well-controlled studies examining the functional role antioxidant enzymes play in protecting the heart from ischemia reperfusion injury in transgenic mice which overexpress EC-SOD by use of the human beta-actin promoter. Thus, we will be able to analyze the relative cardioprotective effects of extracellular SOD to ischemia reperfusion injury. In addition, we will use transgenic EC-SOD knock-out mice to further elucidate the protective contribution extracellular antioxidants play in this model. We hypothesize that overexpression of EC-SOD in transgenic mice will attenuate myocardial ischemia reperfusion injury while mice which have the EC-SOD gene inactivated will be more susceptible to injury. Further understanding of the role antioxidants play in myocardial ischemia reperfusion injury is necessary if current methods of myocardial protection are to be improved upon in a sound and logical manner.